Lens stop or diaphragm



' March 27, 1928. 1,664,137

E. STEWART ET AL LENS STOPOR DIAPHRAGM Filed March 25,. 1927 [/V VE/V 717R 5 W W f4" m Patented Mar. 27, 1928.

UNITED STATES PATENT OFFICE.

EDWARD STEWART AND JOHN GORDON WARREN, F HAMILTON, ONTARIO, CANADA.

LENS STOP OR DIAPHRAGM.

Application filed March 25, 1927. Serial No. 178,451.

Our invention relates to improvements in lens stops or diaphragms with particular reference to photo-engraving or lithographic work and the object of the invention is to de vise a novel form of lens stop or diaphragm for lenses used in process cameras for making half-tone negatives, the use of which will close up or eliminate graduations of tone in the high lights and do away with the necessity of eliminating these high lights subsequently by hand upon the negative, engraved plate or lithographic plate as has hitherto been the case, thus greatly simplifying and expediting the processes of photoengraving and photo-lithography.

novel form of lens stop or diaphragm and in which the various parts are indicated by ref erence characters.

In the form illustrated our improved lens stop is cut in a diaphragm 1 which diaphragm is adapted to be inserted into position with respect to a lens in any suitable way, but as our present invention relates solely to the diaphragm itself and not to any particular manner of mounting it upon the lens, the diaphragm only is illustrated.

To form our improved stop, a series of slots are cut in the diaphragm 1 and these slots are disposed in a particular manner as follows.

The slots are disposed radially in two pairs 22 and 3-3 intersecting at a common centre 4. I

The slots 2--2 and 3-3 of each pair intersect each other at angles of 30 degrees which angles are indicated by the letters A, A and B, B respectively, while the lines 55 and 6-6 bisecting these angles A and B respectfully intersect each other at right angles.

In using our stop it is essential that it be mounted upon the lens in a definite relative position with respect to the lines upon the half-tone screen, which screen as is known by those skilled in the art, bears two series of parallel lines intersecting each other at 90 degrees and, if the half-tone screen is placed so that these lines are at degrees to the horizontal, then our stop must be mounted in the position illustrated, in which one of the lines 55 or 66 is vertical and the other horizontal, as it has been found that with our stop in this relative position with respect to the lines on the half-tone I screen, the lighter portions or high lights of half-tone negatives are closed, so that no subsequent hand work is necessary upon the negatives, engraved plates or lithographic plates in order to eliminate the high lights.

Our stop may be so mounted within the lens that it may be moved angularly about its centre so that, in case the lines of the half-tone screen are positioned other than at 45 degrees to the horizontal as above stated, our stop may be turned into such a position that the slots 2 and 3 occupy the desired critical position relative to the lines on the half-tone screen above referred to.

From the foregoing it will be apparent that we have devised an improved type of stop for the purposes stated whereby the objects of our invention have been attained.

What we claim as our invention is:

A lens stop comprising a diaphragm formed with a plurality of radial slots intersecting at a. common centre, said slots arranged in two pairs, the slots of each pair intersecting each other at 30 degrees, and the lines bisecting the angles of intersection of the slots of each pair being perpendicular to each other.

EDWARD STEW'ART. JOHN GORDON WARREN. 

